<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
 
</head>
<body>
  <div id="app">
    firstname:
    <input v-model="firstname" type="text">
    lastname
    <input v-model="lastname" type="text">

    fullname
    =>
    {{ fullname }}
  </div>
  <script src="./vue.js"></script>
  <script>
    const vm = new Vue({
      el: '#app',
      data: {
       firstname:'',
       lastname:''
      },
      computed: {
        fullname(){
          return this.firstname+"."+this.lastname;
        }
      }
    })

  
  </script>
</body>
</html>